In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Depending on the particular printing member and imaging conditions, certain performance limitations may be observed. For example, a silicone-surfaced dry plate may exhibit insufficient retention of ink by the exposed ink-receptive layer. The source of this behavior, however, is complex; it does not arise merely from stubbornly adherent silicone fragments. Simple mechanical rubbing of the silicone layer, for example, reliably removes from the ink-accepting layer all debris visible even under magnification, and well before damage to the non-imaged silicone areas might occur. Nonetheless, such plates still may print with the inferior quality associated with inadequate affinity for ink. And while ink acceptance can be improved through cleaning with a solvent, this process can soften the silicone as well as degrade its anchorage to non-imaged portions of the plate. Solvents also raise environmental, health and safety concerns. Similar limitations may be observed in wet lithographic printing members having hydrophilic surface layers disposed over oleophilic sublayers.
Study of the imaging process and its effect on certain types of plate constructions, particularly those containing thin-metal imaging layers below silicone top coatings, suggests that the observed printing deficiencies arise from subtle chemical and morphological changes induced by the imaging process. Because the metal imaging layer is in contact with the chemically complex silicone layer, the high temperatures attained during imaging can induce unwanted thermal reactions that produce silicone-derived products. These breakdown products may interact both chemically and mechanically with the underlying ink-receptive substrate surface. That surface, moreover, is also rendered more vulnerable to interaction with silicone breakdown products as a result of exposure to high temperatures, which can melt and thermally degrade the surface of the substrate so that it readily accepts breakdown products. In both dry and wet plate constructions, the adhesion, implantation, mechanical intermixture, and chemical reaction of breakdown products from the surface layer with the underlying oleophilic layer(s) interferes with the printing member's ability to retain ink.
In addition, existing lithographic printing members have the propensity to build up static charge during the coating, handling, cleaning, and printing processes. This can lead to static discharge which poses health and safety hazards. Finally, while on press, the act of “dry rousting” (i.e., rubbing the surface with a dry rubber roll to remove the bulk of the imaging debris), or, in the case of dry printing members, the simple act of printing without water, can also build up a static charge. Spark discharges can jump from one conductive non-image area to another, causing non-conductive image areas to ablate post-imaging, thus creating additional unwanted ink-receptive areas.